The purpose of this research study is to compare how the body reacts to different strengths of an H5N1 flu vaccine when given by different routes of injection (injection into the skin or into the muscle). It will also compare how antibodies (proteins produced by the body's immune system that recognize and help fight infections and other foreign substances in the body) are made after receiving the H5N1 flu vaccine. Participants will include 100 healthy adults, ages 18-40 years. Study procedures include blood samples and completing a memory aid documenting daily oral temperature and side effects for 7 days following each vaccination. Participants may be involved in the study for up to 13 months.